Do you become less fertile after your first baby?

In general, no: A major factor in subfertility is a mom's age. So if you are fairly young when you have your first and try for another when you are still young, you should be fine. If you were older when trying for the first (say late 30s) then with each year ovarian function declines significantly. Also: if you had pregnancy complications (like retained placenta) that could decrease chances in the future.